This application is the national phase under 35 U.S.C. §371 of PCT International Application Ser. No, PCT/W2009/003253 which has an International filing date of Jul. 10, 2009 and designated the United States of America.
1. Field of the Invention
The present invention relates to a communication system for sending and receiving data among a plurality of communication apparatuses. Furthermore, the present invention relates to a communication apparatus that can utilize simple configurations to determine a generating time point of received data, a communication system including the communication apparatus, and a communication method.
2. Description of Related Art
It is recently known in several fields to utilize a system having many communication apparatuses, in which each communication apparatus is provided with a certain function and connected with other communication apparatuses in order to perform many operations through the data exchange between many communication apparatuses. For example, it is known in a field of local area network (LAN) mounted on a motor vehicle to utilize electronic control units (ECUs) as the communication apparatuses, to make each ECU perform a certain operation and to exchange data between the ECUs, in order to provide many functions to be implemented by a system.
Generally, available period is applied to each data in the case of performing a process for exchanging data whose contents have to be changed momentarily among a plurality of communication apparatuses. It may cause a problem if an extremely old data is utilized, for example, by an ECU of an on-vehicle LAN field which controls safety functions.
A patent document 1 shows that each ECU controls to send and receive data in accordance with a priority order based on arrival expiry time for minimizing a number of data which arrive after the arrival expiry time. In the patent document 1, absolute time is utilized for obtaining the arrival expiry time and each ECU includes time clock means for clocking time based on reference time which is synchronized among ECUs in order to send preferentially the data whose arrival expiry time comes earlier among data to be sent.
[Patent document 1] Japanese Patent Application Laid-Open No. 2007-295081